This invention relates to an objective system including a basic objective lens of the ordinary photographic type followed by an attachment lens to extend the focal length of the basic objective lens and, more particularly, to improvements of such attachment lens.
It is known to provide an objective system including a basic objective of fixed focal length either preceeded or followed by an attachment lens of fixed focal length to thereby extend or shorten the focal length of the basic objective. The attachment lens adapted for use at the front of the basic objective is with disadvantage increased in bulk and weight, while the attachment lens adapted for use at the rear, though having an advantage of facilitating further minimization of the bulk and weight compared with the former type attachment lens, presents a more difficult problem in order to achieve good correction of aberrations and, particularly, of field curvature of image.
To overcome the above mentioned drawbacks of such attachment lens, one solution has been proposed by the present applicant in Japanese Patent Publication No.Sho 51-12421. According to this proposal, the attachment lens is divided into two main groups of lens elements, the front group being of negative power and the rear group being of positive power, characterized in that the position of the front principal point is moved further ahead, and each of the lens groups is constructed from strong positive and negative lenses to make use of a low refractive index glass for the positive lens and of a high refractive index glass for the negative lens, whereby an improvement of the Petzval sum is effected.